Small-scale heterogeneity of microbial N uptake in streams and its implications at the ecosystem level.

ABSTRACT

Large-scale factors associated with the environmental context of streams can explain a notable amount of variability in patterns of stream N cycling at the reach scale. However, when environmental factors fail to accurately predict stream responses at the reach level, focusing on emergent properties from small-scale heterogeneity in N cycling rates may help understand observed patterns in stream N cycling. To address how small-scale heterogeneity may contribute to shape patterns in whole-reach N uptake, we examined the drivers and variation in microbial N uptake at small spatial scales in two stream reaches with different environmental constraints (i.e., riparian canopy). Our experimental design was based on two ¹5N additions combined with a hierarchical sampling design from reach to microhabitat scales. Regardless of the degree of canopy cover, small-scale heterogeneity of microbial N uptake ranged by three orders of magnitude, and was characterized by a low abundance of highly active microhabitats (i.e., hot spots). The presence of those hot spots of N uptake resulted in a nonlinear spatial distribution of microbial N uptake rates within the streambed, especially in the case of epilithon assemblages. Small-scale heterogeneity in N uptake and turnover rates at the microhabitat scale was primarily driven by power relationships between N cycling rates and stream water velocity. Overall, fine benthic organic matter (FBOM) assemblages responded clearly to changes in the degree of canopy cover, overwhelming small-scale heterogeneity in its N uptake rates, and suggesting that FBOM contribution to whole-reach N uptake was principally imposed by environmental constraints from larger scales. In contrast, N uptake rates by epilithon showed no significant response to different environmental influences, but identical local drivers and spatial variation in each study reach. Therefore, contribution of epilithon assemblages to whole-reach N uptake was mainly associated with emerging properties from small-scale heterogeneity at lower spatial scales.